It is known that an iso-thiocyanate contained in a mustard or a wasabi (i.e., Japanese horseradish) have bacteriostatic and bactericidal actions, and any of synthetic and natural iso-thiocyanates provides bacteriostatic and bactericidal effects even at an extremely small concentration of vapor thereof on the order of 10-odd ppm to hundreds ppm. Before now after reporting of this fact, several attempts have been made to adsorb the iso-thiocyanate to a porous carrier such as clay minerals or to a surface of a film or a sheet to industrially utilize the bacteriostatic and bactericidal actions thereof, and a certain article made in such a manner, e.g., a freshness maintaining material has been brought to market.
In the above prior art, however, it is impossible to utilize the bacteriostatic and bactericidal actions of the iso-thiocyanate profitably. This is because much of the iso-thiocyanate adsorbed in these methods volatilizes off in the course of production of the article or during storage of the article, because of its high volatility, and consequently, an effective amount of iso-thiocyanate cannot be supported on a carrier. Particularly, when the iso-thiocyanate is adsorbed to a porous carrier, it is necessary to take a measure that the iso-thiocyanate supported on a porous carrier is placed into a sack having a gradully releasing property, because the releasing rate largely depends upon the temperature. When the iso-thiocyanate is adsorbed to a film or a sheet, there is encountered a problem that it is difficult to impregnate the film or the sheet with an amount of iso-thiocyanate enough to exhibit an efficacy, but also it is difficult to control the gradually releasing.
The freshness maintaining means which has conventionally been used is a vacuum pack, or Ageless (Trademark) which is a deoxygenator consisting essentially of active iron oxide, for preventing the oxidation and the propagation of bacteria. However, with the vacuum pack, a special apparatus is required, and with the Ageless, it is necessary to pack the Ageless together with a material to be packed. Many food products such as breads, fishes, meats, box lunches, etc., are only packed with a film such as polyvinyliden chloride and polyethylene (PE) films, or only placed into a container made of a polyvinyliden chloride or a polyethylene. In this case, they are, of course, not durable at room temperature and hence, it is necessary to store them, for example, in a refrigerator.
Thereupon, it is conceived to utilize a freshness maintaining material made using the iso-thiocyanate, but this is also accompanied by the above-described problems, resulting in a difficulty to put it to practical use.